Conventional articles of footwear include two primary elements—a laminated sole attached to a soft and pliable upper. The laminated sole structure generally incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin, cushioning member located within the upper and adjacent to the plantar (lower) surface of the foot to enhance footwear comfort. The midsole, which is traditionally attached to the upper along the entire length of the upper, forms the middle layer of the sole is structure and serves a variety of purposes which include controlling foot motion and providing cushioning. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear-resistant material that includes texturing to improve traction.
The primary element of a conventional midsole is a resilient, polymer foam material, such as polyurethane, ethyl vinyl acetate (EVA), a plastomer, or a rubber compound that extends throughout the length of the footwear. The properties of the polymer foam material in the midsole are primarily dependent upon factors that include the dimensional configuration of the midsole and the specific characteristics of the material selected for the polymer foam, including the density of the polymer foam material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and energy absorption properties may be altered to meet the specific demands of the activity for which the footwear is intended to be used.
Various materials, including leather, rubber, and textile, may be utilized in manufacturing the upper structure. Different layers of the upper may be formed from numerous material elements that each imparts different properties to specific areas of the upper.
Once separately finished, the upper and the sole can be sewn together, glued together, and/or welded together.
U.S. Pat. No. 8,240,067 relates to a shoe, especially to a sport shoe, with a shoe upper and a sole consisting of at least two parts. To obtain a shoe having high flexibility and a low weight, the shoe upper as well as the parts of the sole consist of a material with a low bending stiffness. The shoe upper consists preferably of leather, artificial leather, and/or textile material, while the sole parts can consist of rubber, polyurethane, polyamide, ethyl vinyl acetate (EVA), or a mixture of these materials. To provide the shoe with the necessary stiffness for a usual use, a sole element with a high bending stiffness, which is adapted to the shape of the foot, can be arranged in the interior of the shoe above the parts of the sole. The sole element can consist of a forefoot part and a rearfoot part, wherein the two parts are connected by a hinge or an articulation. The hinge can be designed as a plastic joint which is directly injection molded to both parts. Furthermore, the hinge can consist of polyurethane, polyamide, polyethylene, polypropylene, or a mixture of these materials. The sole element can be provided with reinforcing fibers, wherein primary glass fibers or carbon fibers are suggested.
U.S. Pat. No. 8,245,417 discloses a vapor-permeable and waterproof sole for shoes, including a tread including a plurality of through holes, a multilayer pack, arranged superimposed on the region where the through holes are provided and including a membrane, permeable to water vapor and impermeable to liquids, and a protective layer, arranged directly below the membrane. The pack includes a vapor-permeable or perforated layer element, which is constituted by one of the following materials: nonwoven fabric or pile fabric or needle felt, leather, or microporous EVA.
U.S. Pat. No. 8,296,974 relates to a thermoplastic polyurethane (TPU) molded shoe sole comprising: (a) from about 30 to about 55 weight percent based on total TPU of hard segment comprising structural units of (i) diisocyanate and (ii) chain extender; (b) from 45 to about 70 weight percent based on total TPU of soft segment comprising structural units of: (i) polycaprolactone diol and (ii) poly(oxypropylene)-poly(oxyethylene)glycol polyether diol. This TPU is said to have good combinations of light transmission and clarity, tensile strength, low temperature flexibility, and high moisture vapor transition rates compared to other commercial or known TPUs.
One problem associated with traditional materials for manufacturing footwear soles, such as polyurethane or EVA, is that they cannot deliver a targeted combination of desired properties with a single composition. In addition, in order to adapt to specific requirements on performance of the upper structure, different materials are applied to the upper than those to the sole. Therefore, there remains a need for a material for manufacturing footwear soles which can provide a balance between a set of desired properties including relatively low density, low compression set, weldability, slip resistance, and abrasion durability comparable to those of rubber, etc., preferably to be used for the entire article of footwear so that subsequent unified recycling is possible. Applicants have found that the above can be achieved by manufacturing footwear soles, preferably the entire article of footwear, using a composition comprising a propylene-based elastomer, preferably in combination with an ethylene-based copolymer. Furthermore, the addition of the propylene-based elastomer can improve the weldability, while maintaining instead of compromising other properties at their desired levels, and can reduce or eliminate the use of an adhesive for the bonding process of the upper and the sole by using welding only.